Electrocatalyst design and optimization strategies continue to be an active area of research interest for the applied use of renewable energy resources. The electrocatalytic conversion of carbon dioxide (CO₂) is an attractive approach in this context because of the added potential benefit of addressing its rising atmospheric concentrations. In previous experimental and computational studies, we have described the mechanism of the first molecular Cr complex capable of electrocatalytically reducing CO₂ to carbon monoxide (CO) in the presence of an added proton donor, which contained a redox-active 2,2′-bipyridine (bpy) fragment, CrN₂O₂. The high selectivity for CO in the bpy-based system was dependent on a delocalized Cr^II(bpy^•–) active state. Subsequently, we became interested in exploring how expanding the polypyridyl ligand core would impact the selectivity and activity during electrocatalytic CO₂ reduction. Here, we report a new CrN₃O catalyst, Cr(tpy^tbupho)Cl₂ (1), where 2-(2,2′:6′,2″-terpyridin-6-yl)-4,6-di-tert-butylphenolate = [tpy^tbupho]⁻, which reduces CO₂ to CO with almost quantitative selectivity via a different mechanism than our previously reported Cr(^tbudhbpy)Cl(H₂O) catalyst. Computational analyses indicate that, although the stoichiometry of both reactions is identical, changes in the observed rate law are the combined result of a decrease in the intrinsic ligand charge (L₃X vs L₂X₂) and an increase in the ligand redox activity, which result in increased electronic coupling between the doubly reduced tpy fragment of the ligand and the Cr^II center. The strong electronic coupling enhances the rate of protonation and subsequent C–OH bond cleavage, resulting in CO₂ binding becoming the rate-determining step, which is an uncommon mechanism during protic CO₂ reduction.

The impact of the exact spatial arrangement of the alkali metal on the electronic properties of 9-carbene-9-borafluorene monoanions is assessed, and a series of [K][9-CAAC-9-borafluorene] complexes (1–4) have been isolated (CAAC = cyclic(alkyl)(amino) carbene, (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene). Compound 1, which contains [B]–K(THF)₃ interactions, is compared to charge-separated 2–4, which were prepared by capturing the potassium cations with 18-crown-6, 2.2.2-cryptand, or 1,10-phenanthroline. Notably, the ¹¹B NMR spectra of charge-separated borafluorene monoanions 2–4 show distinct low-field signatures compared to 1. Theoretical calculations indicate that charge separation may be exploited to influence the nucleophilic and electron transfer properties of 9-carbene-9-borafluorene monoanions. When [K(2.2.2-cryptand)][9-CAAC-9-borafluorene] (3) is reacted with 9,10-phenanthrenequinone and 1,10-phenanthroline-5,6-dione, the carbene ligand is displaced, and new air-stable R₂BO₂ spirocycles are formed (5 and 6, respectively). Remarkably, compounds 5 and 6 display fluorescence under UV light in both the solid and solution phases with quantum yields of up to 20%. In addition, a drastic red-shift in the emission color is observed in 6 because of the presence of the nitrogen atoms on the phenanthroline moiety. Mechanistic insights into the formation of these spirocycles are also described based on density functional theory calculations.

The ligand influence on olefin hydrogenation using four capping arene ligated Rh(I) catalyst precursors (FP)Rh(η²-C₂H₄)Cl {FP = capping arene ligands, including 6-FP (8,8′-(1,2-phenylene)diquinoline), 6-^NPFP (8,8′-(2,3-naphthalene)diquinoline), 5-FP (1,2-bis(N-7-azaindolyl)benzene) and 5-^NPFP [2,3-bis(N-7-azaindolyl)naphthalene]} has been studied. Our studies indicate that relative observed rates of catalytic olefin hydrogenation follow the trend (6-FP)Rh(η²-C₂H₄)Cl > (5-FP)Rh(η²-C₂H₄)Cl. Based on combined experimental and density functional theory modeling studies, we propose that the observed differences in the rate of (6-FP)Rh(η²-C₂H₄)Cl and (5-FP)Rh(η²-C₂H₄)Cl-catalyzed olefin hydrogenation are most likely attributed to the difference in the activation energies for the dihydrogen oxidative addition step. We are unable to directly compare the rates of olefin hydrogenation using (6-^NPFP)Rh(η²-C₂H₄)Cl and (5-^NPFP)Rh(η²-C₂H₄)Cl as the catalyst precursor since (5-^NPFP)Rh(η²-C₂H₄)Cl undergoes relatively rapid formation of an active catalyst that does not coordinate 5-^NPFP.

We report carbon-hydrogen acetoxylation of nondirected arenes benzene and toluene, as well as related functionalization with pivalate and 2-ethylhexanoate ester groups, using simple copper(II) [Cu(II)] salts with over 80% yield. By changing the ratio of benzene and Cu(II) salts, 2.4% conversion of benzene can be reached. Combined experimental and computational studies results indicate that the arene carbon-hydrogen functionalization likely occurs by a nonradical Cu(II)-mediated organometallic pathway. The Cu(II) salts used in the reaction can be isolated, recycled, and reused with little change in reactivity. In addition, the Cu(II) salts can be regenerated in situ using oxygen and, after the removal of the generated water, the arene carbon-hydrogen acetoxylation and related esterification reactions can be continued, which leads to a process that enables recycling of Cu(II).

A combined synthetic and theoretical investigation of N-heterocyclic carbene (NHC) adducts of magnesium amidoboranes is presented, which involves a rare example of reversible migratory insertion within a normal valent  s -block element . The reaction of (NHC)Mg(N(SiMe 3 ) 2 ) 2  ( 1 ) and dimethylamine borane yields the tris(amide) adduct (NHC-BN)Mg(NMe 2 BH 3 )(N(SiMe 3 ) 2 ) ( 2 ; NHC-BN = NHC – BH 2 NMe 2 ). In addition to Me 2 N=BH 2  capture at the  NHC C–Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe 2 BH 2 NMe 2 BH 3 ) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe 2 BH 3 ) 2 ( 8 ) and (NHC)Mg(NMe 2 BH 2 NMe 2 BH 3 ) 2  ( 9 ) were synthesized, and a dynamic migration of Me 2 N=BH 2  between Mg–N and  NHC C–Mg bonds was observed in  9 . This unusual reversible migratory insertion is presumably induced by dissimilar charge localization in the ˉ{NMe 2 BH 2 NMe 2 BH 3 } anion, as well as the capacity of NHCs to reversibly capture Me 2 N=BH 2  in the presence of Lewis acidic magnesium species.

Electrocatalytic CO₂ reduction is an attractive strategy to mitigate the continuous rise in atmospheric CO₂ concentrations and generate value-added chemical products. A possible strategy to increase the activity of molecular systems for these reactions is the co-catalytic use of redox mediators (RMs), which direct reducing equivalents from the electrode surface to the active site. Recently, we demonstrated that a sulfone-based RM could trigger co-electrocatalytic CO₂ reduction via an inner-sphere mechanism under aprotic conditions. Here, we provide support for inner-sphere cooperativity under protic conditions by synthetically modulating the mediator to increase activity at lower overpotentials (inverse potential scaling). Furthermore, we show that both the intrinsic and co-catalytic performance of the Cr-centered catalyst can be enhanced by ligand design. By tuning both the Cr-centered catalyst and RM appropriately, an optimized co-electrocatalytic system with quantitative selectivity for CO at an overpotential (η) of 280 mV and turnover frequency (TOF) of 194 s⁻¹ is obtained, representing a three-fold increase in co-catalytic activity at 130 mV lower overpotential than our original report. Importantly, this work lays the foundation of a powerful tool for developing co-catalytic systems for homogeneous electrochemical reactions.